The present invention relates to the communication of digital data using a rate 5/8 punctured convolutional code, and more particularly to such a code that is obtained by puncturing a "standard" rate 1/2 or rate 1/3 convolutional code.
Pragmatic Trellis-Coded Modulation (TCM) is a combined Forward Error Correction (FEC) coding and modulation scheme that utilizes a well-known and "standard" underlying convolutional code applied to certain bits of M-ary Phase Shift Keyed (MPSK) or M-ary Quadrature Amplitude Modulation (MQAM) symbol mappings. It's utility is in providing increased data rate over bandlimited channels using nearly standard off-the-shelf decoding hardware. Viterbi et. al. proposed this scheme and gave an example of applying the output of the underlying standard rate 1/2, constraint length K=7 encoder as the two-least-significant bits (LSBs) of each 3-bit 8PSK symbol and using an uncoded, or parallel branch, bit to choose the most-significant-bit (msb) of the 3-bit 8PSK symbol. Viterbi, Andrew et. al., "A Pragmatic Approach to Trellis-Coded Modulation," IEEE Communications Magazine, July 1989, pp. 11-19. Viterbi's design takes two information bits to produce three channel bits or one 8PSK symbol (one parallel, uncoded bit and one bit that is rate 1/2 encoded into two bits) giving a code rate of 2/3. This code has been employed as a standard in an Intelsat satellite service. Intelsat Earth Station Standards (IESS) Doc. IESS-310, "Performance Characteristics for Intermediate Data Rate Digital Carriers Using Rate 2/3 TCM/8PSK and Reed-Solomon Outer Coding (TCM/IDR)," Approval Date: Sep. 16, 1996, pp. 14-22.
A later work by Wolf et. al. extends pragmatic coding to higher rates, e.g., a rate 5/6 code is built by puncturing the "standard" rate 1/2, K=7 code to rate 3/4 and applying its output along with two uncoded, parallel branch bits over two 8PSK symbol periods. Wolf, Jack Keil and Zehavi, Ephraim, "P.sup.2 Codes: Pragmatic Trellis Codes Utilizing Punctured Convolutional Codes," IEEE Communications Magazine, February 1995, pp. 94-99. Other references of interest include U.S. Pat. Nos. 5,233,629 ("Method and Apparatus For Communicating Digital Data Using Trellis Coded QAM")to Paik et al., 5,396,518 ("Apparatus and Method for Communicating Digital Data Using Trellis Coding with Punctured Convolutional Codes") and 5,408,502 ("Apparatus and Method for Communicating Digital Data Using Trellis Coded QAM With Punctured Convolutional Codes") to How, and 5,511,082 ("Punctured Convolutional Encoder") to How et al.
Puncturing a code is a method for deleting (i.e., not transmitting) certain encoder outputs in a regular manner such that the code still has good distance properties. This can be accomplished, for example, by deleting one of the two rate 1/2 encoder outputs on every other encoder cycle. In this manner, three output code bits are formed for every two input information bits (or every two encoder cycles) giving a rate 2/3 code. Punctures for the "standard" rate 1/2, K=7 code that provide good Hamming distance are well-known. See, for example, Yasuda, Y. et. al., "High-Rate Punctured Convolutional Codes for Soft Decision Viterbi Decoding," IEEE Transactions on Communications, Vol. COM-32, 1984, pp. 315-319.
Trellis coded modulation (TCM) can be used to achieve increased throughput over a given bandlimited communication channel. The rate 2/3 code delivers 60 Megabits/second (Mbps) when used with an 8PSK symbol for a baud rate of 30 Megasymbols/second (Msps); the 5/6 code delivers 75 Mbps when used with a 30 Msps 8PSK rate. In comparison, a non-TCM scheme using a standard rate 7/8 coded, 30 Msps Quadrature PSK (QPSK) signal delivers 52.5 Mbps. The increased throughput achieved with TCM over standard coded QPSK imposes an increase in the required Signal- or Carrier-to-Noise Ratio (SNR or CNR) for a given Bit Error Rate (BER) performance value termed "threshold". Rate 2/3 pragmatic TCM has a CNR threshold that is slightly worse than standard coded rate 7/8 QPSK but is 3 dB lower than rate 5/6 pragmatic TCM.
Both of the pragmatic TCM example designs (rate 2/3 and 5/6 8PSK) have been proposed for use as standards in satellite transmission of digital television (DTV) signals. These existing pragmatic schemes are found through computer simulation to require a 3 dB difference in the CNR for threshold BER operation. CNR is related directly to the required receiver antenna size, receiver noise figure, and satellite transmit power.
Moreover, past implementations of pragmatic TCM have been performed on the "standard" rate 1/2, K=7 convolutional code using "standard" puncture maps found to be optimum for this code in a Hamming distance sense (i.e., for use in schemes in which the coding is decoupled from the modulation scheme instead of integrated as in TCM). See, e.g., the Yasuda et al. article referred to above.
It would be advantageous to provide other code rates between and above 2/3 and 5/6 that offer varying requirements on threshold CNR and bit rate throughput so as to offer flexibility in operation with various communication channels. It would be further advantageous to puncture the "standard" code and other codes to achieve, e.g., an overall rate 3/4 8PSK TCM code that delivers 67.5 Mbps for a 30 Msps baud rate and has a threshold CNR lying midway between the rate 2/3 and 5/6 pragmatic implementations.
The present invention provides a trellis-coded modulation scheme having the aforementioned and other advantages.